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Virtually all cameras need a lens of some kind to collect the light that is scattered from the surface of an object. The lens reconstructs this scattered light as an image on a light sensitive area behind the lens, normally a CCD or CMOS sensor. The construction of a lens system (lens radii, distances between lenses), the distance between object and lens (=working distance), as well as the distance between the lens and the sensor have an impact on the image. The ratio between object reproduction (=image) and object is called magnification. The following diagram explains the function of a lens (see image).

The selection of the focal length of the lens defines the reproduction scale, i.e. the ratio of the image size on the sensor in relation to the object. Lenses consisting of lens elements with fixed spacing and no focusing units are called fixed focal lenses and have a fixed working distance (MOD). Alternatively fixed focal lenses with a focusing unit are available, allowing slight modifications of the MOD. Due to the fixed
focal length these lenses offer a relatively limited magnification range. As parameters such as working distance and required magnification are usually predefined by the application, most machine vision applications use lenses with fixed focal length.

The use of zoom lenses is much less prevalent in machine vision applications. These lenses change their focal length by moving lens elements which can make them mechanically unstable and not suitable for making accurate, repeatable measurements.

Most lenses used in vision applications are manufactured with metal housings and focus mechanisms which guarantee the stability of the lens providing robust and repeatable measurements. In addition some lenses are available with high shock and vibration characteristics which are suitable for even the harshest environments. It is possible to improve stability through the use of glued lenses.

Selection criteria for the perfect objective

Selecting the correct lens has a direct relationship to the sensor used in the camera. This is usually the first step when assembling the appropriate components for the optimum imaging solution. Parameters such as chip size and pixel size are of major importance.
Thus the size of the camera sensor determines the image circle diameter of the lens: The lens has to be able to illuminate the complete sensor area in order to avoid shading and vignetting. Common sensor sizes for imaging and machine vision range from 1/4“ (3.65 mm x 2.74 mm) and 1.2“ (15.5 mm x 15.5 mm), in addition to small field format with 24.0 mm x 36.0 mm which is also used frequently. Many manufacturers offer lenses suitable for these sensor sizes.

Furthermore, the lens has to be able to resolve the pixel size. This size varies from camera to camera: the smaller the pixel size, the better the optical resolution of a lens required. The optical quality of a lens is specified by the MTF-value of the lens. The modulation transfer function (MTF) is the quantitative description of the image quality of a lens, considering all aberration.

The ideal lens would produce an image which perfectly matches the object, including all details and brightness variations. In practice this is never completely possible as lenses act as low pass filters. The amount of attenuation of any given frequency or detail is classified in terms of MTF and this gives an indication of the transfer efficiency of the lens. As a brief explanation, large structures, such as coarsely spaced lines, are generally transferred with relatively good contrast. Smaller structures, such as finely spaced lines, are transferred with low contrast. For any lens there is a point at which the modulation is zero. This limit is often called the resolution limit and is usually quoted in line pairs per millimetre (lp/mm), or with some macro lenses in terms of the minimum line size in ?m.

It is important to consider the system as a whole when specifying resolution. Many modern megapixel cameras use small sensor sizes to reduce costs. However, these small sensors have very small pixels and thus need higher quality and therefore more expensive optics in order to resolve down to these smaller pixels. Sometimes it may be beneficial to select a more expensive megapixel camera with larger pixels that requires less demanding optics, which could reduce the overall system cost for some applications.

Object resolution and reproduction scale

The object resolution is another crucial criterion when selecting the appropriate lens: In order to resolve the details of an object and to ensure definite edge detection, the detail should be reproduced across about 4 pixels. Thus the required reproduction scale is dependent on the required object resolution and the respective pixel size.

Other parameters for choosing the right objective for a new imaging system include the focal length, the field of view or magnification, the F-number and depth of field as well as the working distance and the tolerable aberrations which vary from application to application. At this point it is necessary to verify defect aberration, chromatic and spherical aberration, spatial distortion, spectral transmission and coatings of the required lens and examine whether the entire imaging system can meet the requirements.

Only the knowledge of all these application specific parameters permits the correct selection of the optimum lens to meet the required field of view and image quality.

The easy way to the perfect solution

The variety of all the technical differences of lenses mentioned above makes it very hard for inexperienced users to choose the perfect product for their specific application. But users have an easier option than a time-consuming process of trial and error: STEMMER IMAGING is Europe's largest technology and service provider for the image processing industry, stocking among other things a wide range of
optics from the world's leading manufacturers.

The company covers all important lens types including standard and general- purpose lenses, macro lenses, lenses with fixed and variable focus lengths, zoom lenses, lenses for multi-chip cameras as well as high-precision lenses and large format lenses. Besides this, STEMMER IMAGING offers special optical components such as telecentric lenses and the appropriate accessories such as filters. With more than 35 years of imaging experience, our experts can support their customers in a solution-oriented and reliable way in finding the best optical solution for their imaging task.

Imaging and Vision Handbook

This article summarises content from STEMMER IMAGING’s „Imaging and Vision Handbook“. The second edition of this 450-page book is available from www .stemmer-imaging.de, info@stemmer-imaging.de or can be ordered free of charge by calling phone number 0049 89 809020.